a) Field of the Invention
The present invention relates to an electronic image pickup apparatus comprising an optical system for forming images of an object on light receiving surfaces of image pickup devices which sample and pickup light intensities on the light receiving surface thereof.
b) Description of the Prior Art
An electronic image pickup apparatus, for example a television camera, comprises solid-state image pickup devices such as CCD image sensors and an imaging optical system which functions to form images of an object on light receiving surfaces of these image pickup devices. The image pickup devices of this type comprise a large number of picture elements which are disposed in lattice patterns and configured so as to spatially sample, at discontinuously distributed picture elements, the images of the object formed on the light receiving surfaces of the image pickup devices. These image pickup devices have a function to convert the sampled light intensities into electrical signals and output these signals. The images of the object are reproduced on a display unit such as a monitor television set on the basis of the electrical signals obtained with the image pickup devices.
When the images of the object contain components which have spatial frequencies exceeding a Nyquist frequency limit for the image pickup devices, the image pickup apparatus equipped with the image pickup devices for spatially sampling the images of the object allows spurious signals such as moire or aliasing to be produced, thereby remarkably degrading quality of the images which are reproduced on the display unit. For this reason, an imaging optical system which is to be used in the image pickup apparatus of this type is configured so as to exhibit an effect to limit spatial frequency response so that components having frequencies in the vicinity of the Nyquist frequency limit are eliminated out of spatial frequency components contained in the images of the object (the effect to limit the spatial frequency response will hereinafter be referred to as an optical low pass effect). The optical low pass effect is obtained, for example, by forming dualized images with a quartz filter disposed in the imaging optical system or blurring the images of the object with a phase filter.
Further, other methods are known to obtain the optical low pass effect: Japanese Utility Model Kokai Publication No. Sho 63-24,523 discloses a method to form dualized images by disposing a wedge-shaped prism in the imaging optical system so that a portion of a light bundle passes through this prism; Japanese Patent Kokai Publication No. Sho 47-38,001 proposes a method to form ring-shaped blurred images with a conical prism disposed in the imaging optical system; and Japanese Patent Kokai Publication No. Sho 63-6,520 discloses a method to blur the images by producing spherical aberration of higher orders in the imaging optical system.
Out of the methods mentioned above, the one which is utilized most frequently at the current time is the method which uses the quartz filter disposed in the imaging optical system and exhibits favorable optical low pass effect.
However, the quartz filter exhibits the optical low pass effect only in a direction in which rays are separated from one another by the quartz filter. That is to say, the optical low pass effect produced by the quartz filter is one dimensional. In order to obtain the optical low pass effect in two dimensions (for example, in the horizontal scanning direction and the vertical scanning direction on the image pickup devices) with the quartz filter, it is necessary to use a plurality of quartz filters at the same time. Consequently, the imaging optical system must comprise a large space for disposing the plurality of quartz filters and requires a higher manufacturing cost.
Furthermore, another method is available to obtain the optical low pass effect by disposing a phase filter in the imaging optical system. This method does not require a high cost for obtaining the optical low pass effect, but produces an adverse influence on resolution of the imaging optical system which is larger than that produced by the method utilizing the quartz filter.
Moreover, the method which utilizes the spherical aberration of higher orders for obtaining the optical low pass effect permits manufacturing the imaging optical system at a relatively low cost and obtaining the optical low pass effect in the two dimensions. However, rays which pass in the vicinities of an optical axis are scarcely influenced by the spherical aberration of higher orders. Accordingly, the method utilizing the spherical aberration of higher orders cannot eliminate completely the high-frequency components and can hardly exhibit the optical low pass effect which is as favorable as that obtainable by the method using the quartz filters.
In addition, the method which utilizes the conical prism can exhibit the two-dimensional optical low pass effect and does not require reserving a space, in the imaging optical system, which is not so large as that required for the wedge-shaped prism or that for the quartz filters. Further, since the conical prism can separate also the rays passing in the vicinities of the optical axis, this method is free from the insufficient elimination of the high-frequency components which is allowed by the method utilizing the spherical aberration of high orders. However, the method utilizing the conical prism allows a diameter of a minimum circle of confusion to remain substantially unchanged by defocusing, thereby allowing light intensity to be enhanced at a center of light intensity distribution of point images. Consequently, this method allows MTF (Modulation Transfer Function) of the imaging optical system to be enhanced and cannot provide a favorable optical low pass effect. Furthermore, this method allows aberrations produced in the optical system to hinder favorable resolution of the point images.